Recently, mobile devices have become more and more compact and have been provided with many more functions. In accordance with this, it has been desired that cells used as power supplies of mobile devices have larger capacities. Nonaqueous electrolytic secondary cells including lithium ion secondary cells have features of being lightweight while having a high electromotive force and a high energy density. For these features, the demand for nonaqueous electrolytic secondary cells is increasing as driving power supplies of various mobile devices, for example, mobile phones, digital cameras, video cameras, notebook computers, various other mobile electronic devices and mobile communication devices.
A lithium ion secondary cell includes a positive electrode formed of a lithium-containing composite oxide, a negative electrode containing a negative electrode active material for occluding or releasing lithium metal, lithium alloy or lithium ions, and an electrolyte. Conventionally for the negative electrode active material of the lithium secondary cell, carbon materials are mainly used. Recently, it has been proposed to use a material which allows the cell to have a higher capacitance than the carbon materials. Examples of such a negative electrode active material include materials which occlude lithium ions and have a theoretical capacitance density higher than that of carbon of 372 mAh/g, for example, 833 mAh/cm3 or higher. Such materials are, for example, elements such as silicon, (Si), tin (Sn), germanium (Ge), and oxides and alloys thereof. Among these materials, Si, which has a theoretical capacitance density of 4200 mAh/g and costs low, is especially considered promising. Many Si-containing materials, and structures of the Si-containing materials, have been studied.
However, where used as a negative electrode active material, an Si-containing material significantly expands and contracts as a result of occluding and releasing lithium when the cell is charged and discharged. For example, where Si is used as a negative electrode active material, Si is put into a state where a maximum possible amount of lithium ions is occluded (Li4.4Si). In this state, the volume of Si is increased 4.12 times as compared to the volume when the cell is discharged (in the Si state).
Therefore, when a thin film containing a negative electrode active material such as Si, an oxide of silicon or the like (hereinafter, such a thin film will be referred to as an “active material layer”) is deposited on a current collector by, especially, CVD, sputtering or the like to form a negative electrode, the following occurs. The active material layer expands and contracts as a result of occluding and releasing lithium ions, but the current collector hardly expands or contracts. Therefore, as the charge/discharge cycle is repeated, the closeness of the contact between the active material layer and the current collector is reduced. This may possibly cause the active material layer to be detached from the current collector. Or, in the case of a negative electrode for a wound type cell, when the negative electrode active material expands, the following may occur. The current collector may expand beyond the elastically deformable threshold, resulting in the wound electrode assembly buckling. Here, the term “buckle” refers to a phenomenon that the wound electrode assembly is locally depressed toward the center thereof or a part of the electrode assembly is deformed in a waving shape, due to the expansion thereof.
In order to solve the above-described problems caused by the expansion and contraction of the negative electrode active material, the following structure has been proposed: a plurality of column-shaped bodies containing a negative electrode active material (hereinafter, such a body will be referred to as an “active material body”) are located on the current collector to form a space between adjacent active material bodies, so that the expansion stress of the negative electrode active material is alleviated.
For example, Patent Document 1 proposes the following: bumps and dents are provided on a surface of the current collector, and a film of a negative electrode active material is deposited thereon and etched, so that a space for separating the film of the negative electrode active material into minute areas is formed. Patent Document 2 proposes the following method: a mesh is located above the current collector and an active material layer is deposited through the mesh, so that the negative electrode active material is suppressed from being deposited on an area corresponding to the frame of the mesh. Patent Document 3 discloses forming a plurality of column-shaped active material bodies formed of a negative electrode active material such as Sn, Si or the like on the current collector. Patent Document 4 filed by the applicant of the present application proposes vapor-depositing a negative electrode active material on a surface of the current collector in a direction inclined with respect to the normal to the current collector (oblique vapor deposition), so that a plurality of active material bodies are grown in the direction inclined with respect to the normal to the current collector.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-17040
Patent Document 2: Japanese Laid-Open Patent Publication No. 2002-279974
Patent Document 3: Japanese Laid-Open Patent Publication No. 2004-127561
Patent Document 4: Japanese Laid-Open Patent Publication No. 2005-196970